I am going to disagree with everyone, but I think April of Junior year is a BAD idea...especially if you are taking one of the prereqs during that semester. So much of doing well on that test is controlling nerves and you would most likely be extremely stressed out writing it in the middle of the semester.
Obviously if you could take it the summer before your junior year, that would be ideal. But, if you are left with April or August of your junior year, I would do the August. Yes, you may be at a disadvantage timing wise, but I really think people exagerate that disadvantage (I was an August taker myself), and your scores are simply too important. Better to take it in August and do well than take it in April, do poorly, and have to retake it and risk having your applications thrown out before schools see your new, hopefully better, August scores.
On another note: take a review course...worth every dime! It will save you so much time and stress with studying and you will go into the test completely calm and ready.